Colloidal dispersions are used in coatings inter alia to improve mar and scratch resistance, to improve storage stability of the coating compositions, to assist in rheology control of coatings during application to a substrate, and to improve orientation of pigment particles in coatings containing metallic and other effect pigments. The favorable effects of the colloidal particles are due in large part to the very small size of the dispersed particles, which is less than the wavelength of light. This very small particle size can prevent the particles from scattering light, thereby preventing haziness and adverse color effects that can occur from light scattering in an applied coating. The small particle size also can promote stability of the colloidal dispersions as well as the stability of the coating compositions that contain such dispersions.
Some very small particles, for example silica particles, can associate with one another, forming agglomerates which effectively act as large particles in coatings. Therefore, some of the above-mentioned benefits of the small particle size may be lost. Water molecules in an aqueous carrier successfully compete with the neighboring particles for interaction with the polar groups. Although the stability of the suspension can be affected by factors such as pH and the presence of cations, particularly polyvalent cations, the incorporation of aqueous dispersions into aqueous coating compositions is relatively straightforward. However, in organic coatings or coatings with a high level of non-polar components, the particles have an increased tendency to agglomerate. Since many coating compositions are solventborne, it is desirable to provide a means of incorporating these colloidal dispersions of particles without agglomeration of the particles. In response to this need, sols of particles have been developed that can be used in solventborne and waterborne film-forming compositions.
Historically, highly functional resins offered high mar and scratch resistance in coating compositions, but often had poor appearance properties. Lowering functionality often improved appearance, but at the expense of mar and scratch resistance. Low functionality resins were considered less desirable because of multiple poor film properties, including mar and scratch resistance, due to low crosslink density. Likewise, some two-pack compositions (such as acid/epoxy-cured and isocyanate/hydroxyl-cured systems) demonstrate acceptable appearance but with compromised mar and scratch resistance. While mar and scratch resistance of coating compositions may often be improved by the addition of particles, such improvements may be limited to very particular resin compositions and unobserved with others.
It would be desirable to improve the mar and scratch resistance and appearance of existing film-forming compositions using low functionality resins and readily available crosslinking agents without detrimentally affecting other physical properties of the resulting coatings.